The present invention relates to forming metal parts, and in particular, to an improved apparatus for forming a part from a Titanium sheet while it is in a superplastic state.
For many years it has been known that certain metals, such as Titanium and Aluminum, as well as alloys thereof, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus when in a superplastic condition, the metal or metal alloy exhibits low resistance to deformation and may be elongated with controlled thinning. This permits a sheet of such metal to be rapidly formed against dies to achieve desired shapes. Superplastic forming (SPF) may be performed in conjunction with diffusion bonding (DB). Diffusion bonding refers to a metallurgical joining of surfaces of similar or dissimilar metals by holding them in physical contact and applying heat and pressure sufficient to cause commingling of the atoms at the junction. See for example U.S. Pat. Nos. 3,934,441 of Hamilton et al.; 3,927,817 of Hamilton et al.; 4,984,348 of Cadwell; and 5,016,805 of Cadwell.
One conventional technique of SPF is known as diaphragm forming. A relatively large sheet of Titanium is laid horizontally across an upwardly opening steel forming chamber. The chamber is supported in a hydraulic or pneumatic press so that a steel cover can be closed against the chamber from above. The peripheral edges of the Titanium sheet are firmly clamped between the mating edges of the forming chamber and the cover which are provided with a peripheral seal. The sheet is then heated to the appropriate temperature and formed around a ceramic or steel die supported in the lower chamber. The heating is accomplished utilizing electrically powered radiant heating coils. The formation of the Titanium sheet around the die results from the introduction of a protective envelope of Argon gas on both sides of the sheet and the subsequent release of pressurized gas from beneath the sheet. See my U.S. Pat. No. 4,984,348 entitled SUPERPLASTIC DRAPE FORMING.
Heretofore the radiant heating coils in an SPF press have usually been supported by an upper ceramic platen carried by the upper steel cover of the press. By way of example, the coils may comprise helically wound strands of resistance type Nichrome wire. The coils may be approximately 0.30 inches in diameter, with a thirty watts per square inch heating density. Since the Titanium sheet must be heated to a temperature in the range of 1,600.degree.-1,700.degree. F., a significant amount of electrical energy is consumed.
Typically radiant heating elements used in SPF presses have been squeezed into slots formed in the surface of the ceramic platen. In some cases the radiant heating elements have been embedded in fiberous supports. In another design radiant heating coils have been supported between standoffs formed in the ceramic platen. Most often the radiant heating elements are supported above the Titanium sheet in close proximity thereto. Where radiant heating coils are used it is important that the coils not sag and contact the sheet. The problem with mounting the radiant coils in slots is that only a small portion of each coil is exposed for direct radiant heating, thus reducing the overall heating efficiency. Mounting the radiant coils on standoffs is a complex and fragile arrangement requiring that supporting ceramic rods be extended through the interior of each of the coils, reducing the overall heating efficiency. It would be desirable to provide a much more efficient manner of mounting the radiant heating coils in an SPF press.
Because of the high temperatures involved in SPF, the bottom wall of the steel chamber has had a tendency to bow, which sometimes results in fracturing of the ceramic die supported thereon. The chamber must be made of Chrome-Nickel steel to withstand high temperatures, but still ends up having a limited life. Replacing the chamber is both time consuming and costly. The outer walls of the steel chamber are thermally insulated and part of this insulation is provided by a water cooled jacket. The steel chamber has relatively thick walls to withstand internal pressure, and therefore a relatively large mass. This mass is heated by the aforementioned resistance type electrical heating elements inside the SPF press. Each time the press is opened, a tremendous amount of heat is lost, resulting in substantial additional electric power being consumed in order to maintain the high temperatures required. An SPF press having an entirely steel chamber may require approximately twenty four hours to bring up to 1600.degree.-1700.degree. and then to cycle back down to ambient temperature. If the heating could be made more efficient, this cycle time could be substantially reduced, resulting in tremendous energy savings. Also, if the press could be made more efficient, its external steel walls would be cool and would last longer before fatiguing.
My aforementioned U.S. Pat. No. 4,984,348 discloses an SPF press that utilizes an inner ceramic chamber surrounded by an outer steel jacket. The radiant heating coils are embedded in the upper ceramic platen of the press. While this press has demonstrated improvements in cycle time and energy efficiency, still further improvements in these parameters are desirable.